A) Field of the Invention
This invention relates to a device for dehumidifying by cool-drying a gas and comprises a heat exchanger, the primary part of which is the evaporator of a cooling circuit which further comprises a compressor driven by an electric motor, a condensor, an expansion means between the outlet of the condenser and the inlet of the evaporator, a control device for controlling said motor and measuring means coupled thereto, whereby the compressor is bridged-over by a bypass with therein a bypass-closing element or valve and an open/closed valve or element, whereas the secondary part of the heat exchanger forms part of a conduit for the gas and whereby, at the outlet of said heat exchanger, a liquid separator is disposed in said conduit.
Amongst others, such devices are used for drying compressed air.
B) Discussion of the Related Art
Compressed air delivered by a compressor in most cases is saturated with water vapour or, in other words, has a relative humidity of 100%. This means that condensation will occur with the least drop of temperature. Due to the condensation water, corrosion will occur in conduits and instruments and the devices will show a premature wear and tear.
Therefore, the compressed air is dried, which can be performed in the aforementioned manner by means of cool-drying. Also, other air than compressed air or other gases may be dried in this manner.
Cool-drying is based upon the principle that by lowering the temperature, moisture condenses from the air or the gas, after which the condensation water is separated in the liquid separator and after which the air or the gas is reheated, as a result of which said air or said gas no longer is saturated. The heat is removed from the evaporator by means of the cooling circuit.
The same is valid for other gases than air and each time reference is made to air in the following, the same is also valid for other gases than air.
In practice, there is an ISO standard which determines which can be the dew point and the corresponding lowest air temperature for reference conditions.
In order to prevent that the lowest air temperature becomes smaller than 0xc2x0 C. and, as a result, the evaporator would freeze up due to freezing-on moisture, it is a necessary requirement that the evaporator temperature is higher than 0xc2x0 C.
In order to fulfil this requirement, the measuring means can be provided at the inlet of the evaporator and can measure the evaporator temperature, whereas the control device switches the motor of the compressor, which motor is driven at a constant frequency, on and off in function of said temperature. If this evaporator temperature drops too low, said motor is stopped. If the evaporator pressure subsequently rises too high because the expansion valve still is open, the motor is restarted.
Such regulation, however, is very disadvantageous in consideration of the fact that, with a small load, the compressor is switched on and off continuously, whereas also the evaporator pressure and the dew points strongly vary. Moreover, the condensation dryer has to be constructed rather large.
The measuring means can also be provided at the outlet of the secondary part of the heat exchanger and can measure the lowest air temperature (LAT), whereas the control device, if the temperature in the evaporator tends to drop below 0xc2x0 C., switches off the motor of the compressor of the cooling circuit.
In both kinds of devices, the regulation thus is performed by switching the motor on and off, which, especially with a small load, will happen often, which causes considerable wear and tear of the compressor and is disadvantageous.
This disadvantage is avoided by a device as described in the first paragraph, in which the compressor is bridged-over by a bypass.
Such cool-drying device with bypass is described in DE-A-35.22.974.
The motor is fed with a constant frequency, but is switched on and off by means of a control device formed by a pressure switch, in function of the pressure of the cooling fluid measured between the heat exchanger and the compressor.
When the pressure in the suction conduit of the compressor drops below a certain value which, for example, corresponds to a temperature of the cooling fluid of xe2x88x9215xc2x0 C., the motor is switched off, as a result of which an excessive temperature drop in the suction conduit is avoided.
In order to improve the efficiency of the device, the compressor is bridged-over by a bypass in which, apart from the classic bypass-closing element, also a controlled on/off closing element is disposed.
The bypass-closing element is of a known type which is pushed open when the pressure in the bypass at the side of the inlet of the compressor drops below a certain settable value, as a result of which hot gases are suctioned from the compressor.
Said closing element and the set pressure, whereby the spring no longer keeps the closing element shut, are chosen such that with nominal operating conditions of the cooling circuit, the closing element is closed, but with partial and zero load of the compressor, this closing element is open, such that the evaporator pressure with a hysteresis of 0.2 bar is kept at a minimum, and such that the evaporation temperature, which is coupled to the evaporation pressure of the cooling fluid, downstream of the evaporator is at least 0xc2x0 C. in order to prevent the formation of ice in the evaporator.
If exclusively the bypass-closing element would be present in the bypass, this would result in that the compressor remains operating on full load, even with zero-load conditions. In consideration of the fact that the compressor motor is working continuously, the energy consumption even with no or low load therefore is equal to the energy consumption with nominal load as the high and low pressure in the cooling circuit are continuously kept constant, thus resulting in a relatively high energy consumption.
By adding an open/closed closing element into the bypass conduit, as described in DE-A-35.22.974, the efficiency of the device is improved. This additional open/closed closing element is controlled by a thermal switch which is controlled by a temperature measuring means which is disposed in the gas conduit, at the outlet of the heat exchanger. Said closing element is set such that it opens the bypass when the gas temperature at the outlet of the heat exchanger is approximately equal to the temperature at which the moisture in the gas starts to freeze.
When the temperature of the compressed air at said outlet, for example, is higher than 1xc2x0 C., the closing element closes off the bypass, and the full cooling capacity is led over the evaporator, as a result of which the evaporation temperature in the evaporator, at full load, drops to xe2x88x924 to xe2x88x925xc2x0 C., and therefore the temperature at the outlet will drop. As soon as this latter temperature becomes 1xc2x0 C., the closing element opens the bypass, as a result of which the evaporation temperature in the evaporator will rise to, for example, 1.5xc2x0 C., and moisture frozen on the evaporator evaporates again. The compressed air temperature after the evaporator rises again, and at, for example, 2xc2x0 C. the closing element again closes off the bypass, and the motor can apply its entire power on the heat exchanger.
In this embodiment, freezing of the evaporator can be avoided even if the temperature of the cooling fluid temporarily drops below freezing temperature, such that the condensation-dryer can work with a higher load. However, the motor is driven continuously on full speed, such that energy consumption remains relatively high.
The invention aims at a device for dehumidifying a gas by cool-drying which does not show the aforementioned and other disadvantages and with which in a simple manner, without pressure variations in the cooling circuit and without major wear and tear of the compressor and its motor, energy saving can be achieved.
According to the invention, this aim is realized in that the device for cool-drying comprises means for regulating the speed of the motor, whereas the control device controls these means in function of the value measured by the measuring means.
Instead of switching the motor on and off, its speed is adapted. By increasing the number of revolutions of the motor, a larger mass flow rate of cooling liquid can be transferred by pumping and therefore a larger cooling capacity can be delivered.
By the combination of the bypass with bypass-closing element and open/closed closing element with speed-controlled compressor, not only the number of times the motor is stopped and restarted is significantly reduced, but a major energy saving is possible. Other advantages thereof will be explained in the following.
The aforementioned measuring means can be provided at the cooling circuit and can be means for measuring the evaporator temperature or evaporation pressure.
Said measuring means, however, can also be provided at the conduit for the gas, in or upstream of the secondary part of the heat exchanger, and can be means for measuring the lowest gas temperature (LAT) or can be means for measuring the dew point.
Preferably, the means for regulating the speed of the motor consist of a frequency transformer.
In a particular form of embodiment of the invention, the cool-dryer comprises means for measuring the ambient temperature, which means are also coupled to the control device, and whereby this control device is such that it regulates the speed of the motor in function of the value measured by the measuring means as well as in function of the temperature measured by the means for measuring the ambient temperature.
The invention also relates to a method for cool-drying which, in an interesting manner, uses the device according to the invention described in the aforegoing.
Said invention thus relates to a method for cool-drying of gas containing water vapour, whereby this gas is fed through the secondary part of a heat exchanger, the primary part of which is the evaporator of a cooling circuit which also comprises a compressor which is bridged-over by a bypass wherein a bypass-closing element and a controlled open/closed closing element are provided and which is driven by an electric motor, a condenser, an expansions means between the outlet of the condensor and the inlet of the evaporator, and whereby the aforementioned cooling circuit is controlled in function of the load in such a manner that the cooling capacity is adapted without creating the formation of ice in the evaporator, and which is characterized in that the control of the cooling circuit takes place by regulating the speed of the motor and, moreover, by regulating the open/closed closing element such that, under certain conditions, it opens the bypass and, as the bypass-closing element no longer closes off the bypass, gaseous cooling fluid is conducted from the outlet of the compressor back to its inlet, upstream or downstream of the evaporator.